On Tuesday, Michelle Obama became first lady. By yesterday morning, she had a new title, courtesy of the Washington Post: the leader of the fashionable world.

The level of interest in her wardrobe goes way beyond that of recent first ladies. The American people may have been very fond of Laura Bush, but they didn't want to dress like her. Some first ladies matter, sartorially speaking, and some don't, and not since Jackie Kennedy has a first lady mattered this much.

It doesn't hurt that Michelle is young, tall and good-looking. But there is more to it than that. She looks wonderful in a totally different way from any previous first lady, and that matters. The image of a 6ft tall black woman walking down Pennsylvania Avenue and into the White House symbolises the fact that things really can change for the better, and sometimes sooner than we hoped. Right now, that is a message we all want to hear, which is perhaps why we're so obsessed with all things Michelle.

Barack Obama used his inauguration speech to show us he has stomach for the fight ahead, and he expects the same of us. With hindsight, I guess it was dumb to think he was going to stand up there and announce he had found $700bn dollars down the back of the White House sofa, and called in a favour from the Almighty to have the ice-caps refrozen. Still. We want, we need the Obamas to make our hearts sing. And that's where Michelle comes in.

For the ceremony, she dressed herself and her daughters in a riot of glorious, vibrant, unexpected colour. Her coat and dress were a glinting, greenish-gold. (We're calling it lemongrass, or maybe citrine.) They were a daring choice even before she added the avocado leather gloves and clashing bottle-green patent shoes. Malia, the elder daughter, wore periwinkle blue and black, overturning once and for all the old wives' tale that blue and black don't work together. Sasha - the sassy little one - wore tangerine with candy pink, which really shouldn't work but just totally, totally did. The shades were in themselves a neat celebration of a black first family - not many white skin tones could carry off a tangerine scarf, after all - and the contrast with Laura Bush in her cool, patrician seal-grey was positively joyous.

No less telling than the clothes themselves were the labels inside them. Michelle's coat and dress for the ceremony were by Isabel Toledo, a 47-year-old Cuban-American designer who is well known to fashion insiders but has almost no profile among the public. On a day when titans of American fashion from Oscar de la Renta to Donna Karan were falling over themselves to dress her, Michelle chose a designer who is hugely respected within the industry but whose advertising budget stands at zero dollars. When it came to getting changed for the evening, she appeared in a one-shoulder ivory gown by Jason Wu, a 26-year-old Taipei-born, American-based designer whose name was previously known only to close readers of those New Hot Names To Watch lists in fashion magazines.

"Our minds are no less inventive," her husband told us on Tuesday, "than when this crisis began." By showcasing little-known American designers of diverse heritage, the new first lady deftly illustrated both the depth of talent and endeavour in the huge American fashion industry, and her skill as a cheerleader for it. The shoes for both events were Jimmy Choo; the bold jewellery - huge cocktail rings, long drop earrings - an extension of the fondness for statement jewellery she displayed during the election campaign.

It takes a woman who understands the power of clothes to create two fashion moments out of one day, which is what Michelle Obama did. For the ceremony she presented a happy family in a bright colours; for the evening ball, with the children in bed, she gave us the newlywed image of a dark-suited man taking to the dancefloor with his bride in floor-length ivory, on the first day of a new chapter in their lives. The first look is about modernity, optimism for our children's future; the second is about eternal values, vows and promises.

She knows what she's doing, this lady. In fashion, at least, our future is in safe hands.